1. Field of the Invention
The present invention relates to tunable wavelength filters and, more specifically, to closed-loop control of tunable wavelength filters.
2. Technical Background
Tunable filters, e.g., fiber Bragg gratings (FBGs) have been utilized in a number of optical systems to selectively add and/or drop wavelengths (i.e., channels) at appropriate locations in wavelength division multiplexed (WDM) optical systems. As is well known to one of ordinary skill in art, a tunable FBG is a narrow band reflective element which can be tuned on or off an International Telecommunications Union (ITU) standard wavelength such that a channel may be reflected by or transmitted through the FBG. In this manner, FBGs act as selectable notch band stop filters which substantially reflect received signals within a range of wavelengths and substantially pass signals which are not within the range of wavelengths. An ideal FBG reflects one channel of a WDM signal and passes the remaining channels substantially unattenuated.
In a typical optical system, the addition or subtraction of a channel (i.e., a specific range of wavelengths) has been achieved by a controller, which controls a given FBG between one of a transmissive and a reflective state. In such a system, there has typically been a FBG for each channel of the WDM signal and the FBGs have been actuated between a transmissive and reflective state in a number of ways. For example, the period of the grating may be changed by applying a physical stress to the fiber through the use of an actuator, such as a piezo-electric device. In this manner, adjusting the power applied to the piezo-electric device, via a controller, causes the range of wavelengths reflected by an associated grating to change.
Alternatively, the effective refractive index of the fiber waveguide may be thermally tuned such that the range of wavelengths reflected by the grating varies with temperature. In this manner, the temperature of a grating is adjusted by applying an appropriate amount of power to a heater, which is typically made from an electrically resistive coating, that is in thermal contact with the grating. In such systems, each grating has typically been calibrated such that a given grating reflects a given channel at a given temperature. However, in such systems, the ability to switch a grating from one wavelength to another is limited. In the case of a glass fiber both the ability to stretch the fiber and to change its refractive index with temperature are limited. There can also be limitations to switching (tuning) speeds. In temperature controlled systems, the switching speed limitation has generally been attributable to a grating associated thermocouple, which is located near the grating to sense the grating temperature. A controller, coupled to the thermocouple, monitors the temperature reported by the thermocouple and adjusts the power delivered to an associated heater, accordingly. However, the temperature reported by the thermocouple typically differs, at least after an initial change, from the temperature of the heater. As such, the controller may overshoot or undershoot a desired heater temperature multiple times before stabilizing on a desired temperature and, thus, experience difficulty in locking onto a desired channel.
There exists a need, therefore, for a practical closed-loop control system that is capable of maintaining a tunable wavelength filter locked to a desired wavelength. It would also be desirable for such a system to be able to switch the tunable wavelength filter from one channel to another channel in a reliable relatively efficient manner, covering a wide range of wavelengths and performing the wavelength shift in an acceptably short period of time.
The present invention is directed to a wavelength selective optical device for locking to a selected channel in an ITU grid. The wavelength selective optical device includes a first grating, a second grating, one or more heating elements and a control unit.
The first grating comprises a tunable filter that reflects a selected channel from a received wavelength division multiplexed (WDM) signal. The second grating comprises a reference filter that receives a reference signal and provides an indication signal. The heating element is in thermal contact with the tunable filter and the reference filter and the control unit is coupled to the heating element(s) and the reference filter. The control unit varies a temperature of the heating element(s) responsive to the indication signal provided by the reference filter to adjust the selected channel of the tunable filter.
The first and second gratings may be independent gratings formed in a bulk material, in a planar one-dimensional or two-dimensional waveguide or in a fiber. The materials of construction may be any suitable optically transmissive material including silica, doped glasses and polymers.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.